Conventionally, wires and conductive circuits of electronic parts, such as RFID antennas, required to have high reliability, are formed by sputtering an expensive noble metal on a masked substrate. However, in a method for forming wires and conductive circuits by sputtering, it is required to carry out various processes, so that the productivity thereof is not high. In addition, a part of the expensive noble metal input as a raw material is not used for forming the wires and conductive circuits, so that it has been studied to form wires and conductive circuits by other methods from the point of view of the effective use of resources.
In recent years, as a method for easily mass-producing wires and conductive circuits of electronic parts, there are noticed printed electronics for applying a print technique to form wires and conductive circuits, and it has been studied that a conductive ink containing metal particles dispersed in a dispersion medium is printed on a substrate by any one of various print techniques, such as flexographic printing and screen printing, to sinter the metal particles with each other to form wires, conductive circuits or the like.
On the other hand, when the particle diameter of the metal particles is about a few nanometers to about tens nanometers, the surface area of the metal particles is very large, so that the melting point thereof is dramatically lowered. For that reason, as compared with a case where a conductive ink containing metal particles having a particle diameter of about a few micrometers dispersed in a dispersion medium is used for forming wires or conductive circuits, it is not only possible to form fine wires or conductive circuits, but it is also possible to sinter metal particles with each other even if they are burned at a low temperature of not higher than 200° C. Therefore, it is possible to use various substrates, such as a substrate having a low heat resisting property. For that reason, it is expected that a conductive ink (a fine metal particle dispersing solution) containing fine metal particles (metal nanoparticles) having a particle diameter of not greater than tens nanometers is applied to printed electronics to form fine wires or conductive circuits of electronic parts.
Fine metal particles having a particle diameter of not greater than tens nanometers have a very high activation. Such fine metal particles are unstable as they are. For that reason, in order to ensure the independence and preservation stability of the fine metal particles by preventing the fine metal particles from being sintered and/or aggregated with each other, there is proposed a conductive ink (a fine metal particle dispersing solution) wherein fine metal particles coated with an organic matter, such as a surfactant having a long chain, are dispersed in an organic solvent, such as decane or terpineol. However, if fine metal particles are coated with a surfactant having a high molecular weight and a long chain, the boiling point and decomposition point thereof are high. For that reason, when the fine metal particles are sintered with each other to form wires, conductive circuits or the like, it is required to treat the fine metal particles at a high temperature in order to remove and decompose the surfactant on the surface of the fine metal particles, so that it is not possible to use a substrate having a low heat-resistance. In addition, it is required to carry out the heat treatment for a relatively long time of about 30 minutes to about 1 hour, so that the productivity thereof is deteriorated. Moreover, if an organic solvent is used as the dispersing medium of the conductive ink, it may cause the pollution of the environment unless it is carefully disposed. In addition, when the organic solvent is heated or allowed to stand in an open system, evaporated organic components diffuse to the environment, so that it is required to install a local exhaust ventilation or the like when a large amount of organic solvent is treated. Therefore, if it is possible to use a dispersing medium containing no organic solvent as a main component, it is desired in terms of environment and work.
For that reason, there is proposed a silver nanoparticle composition wherein silver nanoparticles protected with an organic acid or a derivative thereof are dispersed in a solvent containing water as a main component (see, e.g., International Publication No. 2012/026033). If this silver nanoparticle composition is used as a conductive ink, even if a heat treatment is carried out at a low temperature for a short period of time, it is possible to sinter the silver nanoparticles with each other to form good wires, conductive circuits or the like on a substrate.
However, if the metal nanoparticles composition disclosed in International Publication No. 2012/026033 is used as a conductive ink to be printed on a substrate in the shape of a complicated wiring design by means of a continuous roll-to-roll flexographic printing machine which is industrially used in general, there is a problem in that a thin web-shaped film is formed between adjacent wiring portions facing each other, so that a short circuit of a wire formed by heat treatment is easily caused. It is considered that such a thin film is caused by a reaction product of the organic acid or derivative thereof and ammonia (added for adjusting pH during the synthesis of silver nanoparticles and/or after the reaction) in the silver nanoparticle composition disclosed in International Publication No. 2012/026033.
It is possible to prevent the formation of such a thin film by using a low-volume anilox roll. However, if the low-volume anilox roll is used, there is a problem in that the transfer ratio of the conductive ink is deteriorated, so that the thickness of a wire formed by heat treatment is decreased to increase the resistance thereof.